The present invention pertains to transformer ballasts for supplying power in suitable form to gaseous discharge devices or the like, and particularly to a construction for an autotransformer ballast whereby a single bobbin may be utilized in fabricating the whole ballast.
In order to provide background material so that the context and applicability of the invention may be fully appreciated, reference may be made to the following U.S. Pat. Nos. 3,247,422 and 3,371,244. In the former, an invention is disclosed involving a transistor inverter ballasting circuit in which an autotransformer is utilized for ballast purposes. A plurality of four windings are incorporated in the autotransformer, one of which serves as a primary and the four windings together constitute the secondary; the load circuit connected across the primary secondary includes a current limiting impedance in the form of a capacitor, and a gaseous discharge lamp. A detailed circuit description in U.S. Pat. No. 3,247,422 refers to the four windings as being constituted of entirely different wire sizes and, of course, having a variety of different turns for the windings. The core described therein is a molybdenum permaloy toroid.
The other reference cited above, namely, U.S. Pat. No. 3,371,244 discloses a ballast circuit for discharge devices such as ultraviolet lamps and indicates a preferred construction for its ballast transformer as involving an open magnetic flux path, and in which the core comprises a stack of rectangular laminations resulting in a core in the form of a straight bar. Such open magnetic flux path construction provides the necessary leakage inductance so that the secondary winding can serve as the ballast for a gaseous discharge tube. As before with U.S. Pat. No. 3,247,422, a variety of different size wires are contemplated for the plurality of windings of the transformer.
The most pertinent known construction is that developed by applicant's assignee, which is depicted in FIG. 1 of the drawing. Such construction will be described in detail hereinafter. Suffice it to say here that the autotransformer involved is for the purpose of supplying power to two 40 watt, 120 volt fluorescent lamps, the autotransformer featuring a main primary winding of a particular wire size, and a main secondary winding of a different wire size connected in series with the first main winding. Additionally, three windings with a much smaller number of turns than the first main and second main windings serve to heat the filaments of the fluorescent lamps. The three filament windings are normally wound on the last layer of the first main winding.